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ArtesDec 11 '12 at 12:53

An alternative is to use Overlay and overlay the list plot with a gradient as per this answer. Keys are to make sure your image size and image padding in both plots match up. In the DensityPlot I'm using the 4th argument to RGBColor to give the varying opacity.

In the above two plots, I've added PlotRangePadding -> None so that the background only appears inside the plot range. But if you want the whole region of the plot to be filled, you can do that by adding PlotRangeClipping -> False instead of suppressing the padding:

Although I use VertexColors in the definition of gradientBackground, I then immediately rasterize it. You could leave that out and try the last plot again, to see how bad it would look without rasterization. To achieve the adjustability of the background, I wrapped the rasterized gradient in Show with the option AspectRatio -> Full, which makes the inset rubbery when I then specify the third argument of Inset to be 1 in ImageScaled coordinates.

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